1. Field of the Invention
The present invention relates to one-pass scan type (line scanning type) color image reading apparatuses capable of eliminating or reducing moiré in the sub-scanning direction.
2. Description of the Related Art
Smaller and manufacturing-cost-saving color image reading apparatuses that light (energize) each of the light sources of RGB in a line sequential order and use an equi-magnification image sensor are achieved. Such color image reading apparatuses are widely used in image scanners and facsimile machines.
The color image reading apparatuses can generate color image data for one line by sequentially lighting each of the light sources of RGB such that an area of an original corresponding to one line is scanned by each of the light sources of RGB equally for ⅓ of the line, and by combining obtained color component signals.
The conventional color image reading apparatuses read an image by emitting lights of R, G and B to an original image and forming an image with the reflected lights on an acceptance surface having a plurality of image pickup devices. In such color image reading apparatuses, an optical system is provided between an imaging part (lens) and the acceptance surface (photo acceptance unit). The optical system includes at least two kinds of low-pass characteristics with respect to the direction in which the photo acceptance unit is arranged.
The ratio of the area scanned by each of the light sources of RGB is equal irrespective of the resolution in the sub-scanning direction of output color image data. For example, as shown in FIG. 1A, the ratio among RGB is 1:1:1 in either a case where color image data of the resolution in the sub-scanning direction of 200 dpi are generated or a case where color image data of 600 dpi are generated. It should be noted that FIG. 1A shows a case where light emitting elements are lighted in the order of R, G and B, however, the same applies to a case where the light emitting elements are lighted in the order of R, B and G.
On the other hand, when performing exquisite color printing, a dot process (screening) is performed as a color expression technique. In the dot process, as shown in FIG. 1B, three colors of ink, cyan (C), magenta (M) and yellow (Y), are arranged in dot states. Various colors are expressed by the existence of dots of each color C, M, Y and the magnitude relation among the diameters.
Generally, the arrangement of dots of each color is determined using visual characteristics of human beings. More specifically, dots of yellow (Y), for which color human beings possess a low visual sensitivity, are laid out in the horizontal/vertical directions in which the visual sensitivity of human beings is high with respect to the angle. Dots of the other colors are laid out in oblique directions at separation angles of 30°.
In accordance with such an arrangement, dot intervals of each color and the size of the diameters of dots determine “smoothness” of a print. As for the dot interval, approximately 200 dpi is widely used.
When such a print is read by the one-pass scan type color image reading apparatus, depending on the resolution, moiré might be generated for a specific color.
For example, a case is examined where a color print in which dots are laid out at an interval of 190 dpi in the sub-scanning direction is read by matching the up and down direction of the original image with the reading direction at the resolution of 200 dpi in the sub-scanning direction. In this case, as shown in FIG. 2, the dot intervals of the other colors are (¾)1/2 and ½ where the dot interval of Y (yellow) is 1 and dots of Y (yellow) are laid out in the vertical direction with respect to so-called raster scanning. The color having the closest dot interval to the interval of the scan resolution is Y (yellow). Accordingly, moiré tends to occur in Y (yellow) (refer to FIG. 10A).
Such a phenomenon tends to occur in an area where dots of Y (yellow) have the size with which size the dots are not coupled. For example, when expressing gray, which is a color of the original, such moiré of Y (yellow) occurs. When moiré of Y (yellow) occurs in the read image of such an area, a periodic solid area of Y (yellow) that does not seemingly exist in the original tends to be conspicuous. Thus, the quality of the print image is degraded. In addition, when the original is read at an angle, moiré as mentioned above might occur with respect to a color other than Y (yellow).
Further, it should be noted that moiré as mentioned above occurs not only when reading with RGB but also when reading a color original image by light sources of the other three colors or more than three colors.
Conventionally, a variety of countermeasures against moiré have been proposed. For example, Japanese Laid-Open Patent Application No. 05-196894 proposes a moiré reducing method of eliminating moiré by using two kinds of optical low-pass filters. However, in this method, the dedicated components (optical low-pass filters) are used, and besides, the method can only be applied to the reading part of a demagnification optical system using a CCD.